1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method, and more particularly, to an image forming apparatus and image forming method comprising a developing apparatus which performs development by transferring a powder by means of a traveling wave electric field.
2. Description of the Related Art
Image forming apparatuses are known, such as printers, facsimile machines, copying machines, plotters, combined printer/fax/copying machines, and the like, which use an electrophotographic process in which an image carrier is charged, a latent electrostatic image is formed thereon, and this electrostatic latent image is developed by depositing powder of a coloring material, or the like, (hereinafter, called “toner” or “toner particle”) on the latent image, whereupon the toner image is transferred to a recording medium (such as a transfer material, paper, recording paper, intermediate transfer body, or the like).
An image forming apparatus of this kind is known, as described for example in Japanese Patent Application Publication No. 3530124 (hereinafter, called “PRIOR ART 1”), in which a developer conveyance device is provided to convey the developer by creating a traveling wave electric field through applying a multi-phase voltage to a plurality of electrodes arranged at a predetermined interval, and in a developer apparatus having a developer conveyance device of this kind disposed in the developing region opposing the image carrier which carries the electrostatic latent image on the surface thereof, the developer conveyance device comprises a plurality of electrodes arranged in the form of an endless loop, the plurality of electrodes being divided into a plurality of units and a device being provided in order to control the voltage applied to each unit, in such a manner that a multi-phase voltage can be applied to each unit, independently.
Furthermore, Japanese Patent Application Publication No. 2004-198675 (hereinafter, called “PRIOR ART 2”) and Japanese Patent Application Publication No. 2002-258601 (hereinafter, called “PRIOR ART 3”), for example, propose a developing apparatus using a developing method which includes horizontal movement (conveyance) and vertical movement (hopping) of powder by means of electrostatic force, an image on the surface of an electrostatic conveyance member being developed by causing powder to jump up with a component of movement in the direction of travel, by means of a transference electric field. This developing apparatus for developing latent images on an image carrier by depositing powder on the image carrier comprises a conveyance member having a plurality of electrodes, disposed facing the image carrier, which generate a traveling wave electric field in order to move the powder. An electric potential of n phases is applied to the electrodes of the conveyance member, thereby forming an electric field in directions which directs the powder toward the image carrier in the image portion of the latent image, but which does not direct the powder toward the image carrier, in the non-image portions.
On the other hand, as described in PRIOR ART 3, an image forming apparatus is also known in which a photosensitive body is positively charges by a corona charger, a negative electrostatic latent image is formed thereon by exposure to light (namely, an electrostatic latent image in which the surface potential of the photosensitive body is reduced in the positions of the image lines that have been exposed to light), a yellow toner image is then formed by developing the latent image in a developing apparatus using a developing roller, whereupon the yellow electrostatic latent image is erased by removing the charge from the photosensitive drum with a de-charging lamp, and the steps of charging, exposure, development and charge removal are repeated in a similar fashion, thereby successively forming toner images of magenta, cyan and black on the photosensitive body. When all of the toner images have been formed in this way, the electrostatic latent image is de-charged optically by means of the de-charging lamp, and the toner images are transferred electrostatically to normal paper, by means of a corona charger. This image forming apparatus is based on a system (known as a color superimposition method, or image-on-image method) in which a corona charge is applied through the toner layer formed on the photosensitive body, thereby charging the photosensitive body, and toner layers of other colors are superimposed on top of the existing toner image. A similar method is described in Japanese Patent Application Publication No. 08-003673 (hereinafter, called “PRIOR ART 4”), for example, which uses a jumping development method as described below.
Other image forming apparatuses are also known, for instance, as described in Japanese Patent Application Publication No. 2002-6578 (hereinafter, called “PRIOR ART 5”) and Japanese Patent Application Publication No. 9-197781 (hereinafter, called “PRIOR ART 6”), an apparatus which performs development by means of a method known as “jumping” development, in which a superimposed DC voltage and AC voltage is applied between an image carrier and a development roller, thereby transferring the toner from the development roller to the image carrier by means of a non-contact method, or as described in Japanese Patent Application Publication No. 9-329947 (hereinafter, called “PRIOR ART 7”) or Japanese Patent Application Publication No. 5-31146 (hereinafter, called “PRIOR ART 8”), an apparatus which uses an electrostatic conveyance plate, conveys toner to a position opposing the image carrier, and causes the toner to vibrate, float or assume smoke-like behavior, the toner being separated from the conveyance surface by means of a force of attraction generated between the toner and the image carrier and being deposited onto the surface of the image carrier, or as described in Japanese Patent Application Publication No. 5-31147 (hereinafter, called “PRIOR ART 9”), an apparatus which uses an electric field curtain.
Furthermore, with respect to the developing bias of the development apparatus, as described in Japanese Patent Application Publication No. 3-21967 (hereinafter called “PRIOR ART 10”), for example, an image forming apparatus is known in which the peak voltage of the developing bias is changed in accordance with the variation of the electrostatic capacitance C2 between an image carrier and a developer carrier, by introducing a capacitor C1 and a resistor R, connected in parallel to each other, in series to the output side of the bias power supply, and furthermore, a variable capacitor C3 is introduced in parallel with the electrostatic capacitance C2.
In an image forming apparatus based on a color superimposition method (image-on-image method) as described above, if a contact type developing apparatus is used, which moves toner to a region opposing an image carrier by means of a developing roller (developing sleeve) that rotates while carrying a two-component developer comprising a magnetic carrier and toner, as described in PRIOR ART 3 above, then the image is disturbed, for instance, due to the preceding toner image formed on the image carrier being disrupted by the magnetic effects created during subsequent development processes, and hence the image quality declines markedly.
Therefore, in an image forming apparatus of this kind, when using a non-contact type developing apparatus, image disturbance is prevented by setting extremely strict conditions, by which L4>L3/10 and L4>10 (μm), where the thickness of the photosensitive body is taken to be L4, 500<|Vs|<1000 (V), where Vs is taken to be the surface potential of the photosensitive body, 10<L1<40 (μm), where L1 is taken to be the thickness of the toner layer A deposited on the photosensitive body, 10<L2<50 (μm), where L2 is taken to be the thickness of the toner layer B on the toner carrier, 50<L3<150 (μm), where L3 is taken to be the interval between the toner layers A and B. Furthermore, the direction of movement of the surface of the toner layer A and the surface of the toner layer B are set to be the same direction of travel and their speeds of movement are set to be substantially the same.
However, since a good image can only be obtained when the surface potential of the image carrier, the thickness of the toner layer on the image carrier, the thickness of the toner layer on the developer carrier, the speed of movement of the surfaces of the toner layers, and the like, comply with extremely strict conditions in this way, then there is a problem in that such as apparatus cannot effectively be put into practice.
Furthermore, in the image forming apparatus described in PRIOR ART 4, color superimposition is carried out by using a jumping development method, but since jumping development is not a unidirectional development method, then similarly to a case where the aforementioned contact-type developing apparatus is used, the toners already deposited on the photosensitive body may be disturbed during development of the other colors, and toners of different colors may become mixed into the developing devices during development, thus giving rise to color mixing, and the like. In addition, jumping development generally requires a development potential of the order of 400-600 V, and when developing a plurality of colors, it is difficult to ensure a suitable differential in the development potential, thus leading to a decline in reproducibility.
A current and future issue in image forming apparatuses using an electrophotographic process is how to meet requirements relating to image quality, cost and the environment. In the case of image quality, when forming a color image, there is the issue of how to develop one isolated dot having a diameter of only 30 μm, in the case of a 1200 dpi resolution, and more desirably, how to be able to develop images without leaving any borders. Furthermore, with regards to costs, in the case of a personal laser printer, for instance, it is important to lower the total cost which includes not only the unit cost of the developing unit and developing agent, but also the costs of maintenance and final disposal. Moreover, in terms of the environment, it is particularly important to prevent the scattering of toner, which consists of very fine powder, either inside or outside the apparatus.
From these perspectives, the EH development method proposed previously in PRIOR ART 2, and the like, which uses an EH (Electrostatic transport & Hopping) development technique in which a powder is made to jump, with a movement component in the direction of travel, on the surface of an electrostatic conveyance member, by means of a phase-shift electric field (traveling wave electric field), is a unidirectional development method which does not involve contact with the image carrier and which provides excellent performance in that it is capable of developing, for instance, an isolated dot of only 30 μm diameter in a 1200 dpi image as described above.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Application Publication No. 2004-198744 and Japanese Patent Publication No. 3,376,199.